1. Field of the Invention
The present invention relates generally to a data transmission method in a mobile communication system, and in particular, to a reverse data transmission method in a mobile communication system.
2. Description of the Related Art
An high data rate mobile communication system is a CDMA (Code Division Multiple Access) system dedicated to transmission of packet data. For efficient packet transmission both on the forward link and on the reverse link, scheduling must be appropriately made.
On the forward link, an AN (Access Network) transmits data to a particular access terminal (AT) showing the most excellent channel state, taking the air state and other channel conditions into consideration, to thereby maximize the data transmission throughput of the AT. On the reverse link, on the other hand, a plurality of ATs access the AN simultaneously. Therefore, the AN controls the data flow from the ATs in order to prevent traffic congestion and controls overhead within its capacity.
Reverse data transmission is controlled by the use of an RAB (Reverse Activity Bit) and an RRL (ReverseRateLimit) message transmitted from an AN in the existing high data rate system. A forward MAC (Medium Access control) channel is transmitted to an AT in TDM (Time Division Multiplexing) along with a pilot channel, an FAB (Forward Activity Bit), and an RAB. The RAB represents the congestion of the reverse link, and a data rate available to the AT varies with the RAB. That is, the AN increases or decreases the data rate of the AT by means of the RAB when controlling overhead and capacity on the reverse link. However, since the RAB is broadcasted, all ATs receiving the RAB indiscriminately double their data rates or reduce them by half according to the RAB. The RAB occurs repeatedly during transmission as shown in Table 1 below. If RABlength is “00”, that is, eight slots, the same RAB occurs repeatedly in eight slots. As the RAB is made longer, the period for which the same RAB lasts is increased, that is, the data rate of the reverse link is changed more slowly.
TABLE 1BinaryLength (slots)008011610321164
While the above reverse data rate control method utilizing RAB provides simply band management in the system aspect, it does not ensure a dynamic data rate for an AT and has a serious influence on the quality of data transmission.
For example, the characteristics of ATs like premium ATs and emergency ATs and data characteristics, such as high quality transmission and real time transmission, are not considered in determining a data rate.
FIG. 1 is a flowchart illustrating a conventional reverse data ate determination operation at an AT in an HDR system.
In general, an AT notifies an AN of the current data rate on the reverse link by an RRI (Reverse Rate Indicator). The reverse data rate is one of 4.8, 9.6, 19.2, 38.4, 76.8, and 153.6 kbps.
At an initial access, the AT transmits packet data with a preamble at a default data rate of 9.6 kbps, in step 100. Upon receipt of an RRL message in step 102, the AT compares the current data rate with a data rate set in the RRL message in step 104. If the current data rate is lower than that of the RRL message, the AT waits 32 slots (53.3 ms) in step 106 and resets the reverse data rate based on the RRL message in step 108.
On the other hand, if the current data rate is higher than the data rate of the RRL message, the AT resets the reverse data rate based on the RRL message in step 110. To help the AT reset the reverse data rate, the AN transmits an RRL message shown in Table 2.
TABLE 2FieldLength (bits)Message ID829 occurrences of thefollowing two fieldsRateLimitIncluded1RateLimit0 or 4ReservedVariable
The RRL message can include up to 29 records, each record indicating a data rate assigned to a corresponding MACindex. MACindexes are numbered 3 through 32. Message ID is a field indicating the ID of the RRL message and RateLimitIncluded indicates whether a field RateLimit is included or not. If RateLimitIncluded is 0, RateLimit is omitted and, if RateLimitIncluded is 1, RateLimit is included. RateLimit indicates a data rate assigned to an AT. The AN can assign the following reverse data rates to ATs in 4 bits.
0 × 0 4.8 kbps0 × 1 9.6 kbps0 × 2 19.2 kbps0 × 3 38.4 kbps0 × 4 76.8 kbps0 × 5153.6 kbpsAll other values are invalid.
In step 112, the AT transmits packet data at a reset data rate. While transmitting the reverse data, the AT monitors forward MAC channels, especially RABs on the forward MAC channels transmitted from ANs in step 114.
FIG. 2 illustrates operations between HDR sectors in an active set and an AT. As shown in FIG. 2, a forward traffic channel, a reverse traffic channel, a forward MAC channel, and a reverse MAC channel are established between the AT and sector 1 connected to the AT. Meanwhile, no forward traffic channels are assigned between the AT and sectors 2 through 6 that are not connected to the AT. The AT can have up to 6 sectors in its active set and monitors RABs on forward MAC channels from all the sectors of the active set to determine a reverse data rate. Consequently, all ATs receiving an RAB from a sector may increase or decrease their data rates indiscriminately.
Returning to FIG. 1, the AT determines whether at least one RAB is set to 1 in step 116. If it is, the AT determines whether the current rate is 19.2 kbps or higher in step 126. If it is, the AT reduces the data rate by half. On the other hand, if the current rate is below 19.2 kbps, the AT maintains the data rate in step 128.
If all the RABs received from the sectors of the active set are 0s in step 116, the AT checks whether the current transmission is an initial access in step 118. In the case of the initial access, the AT applies the initial data rate in step 100. Otherwise, the AT checks whether the current data rate is a maximum one, 153.6 kbps in step 120. If it is the maximum data rate, the AT maintains the current data rate in step 122, and other wise, the AT doubles the current data rate in step 124 and transmits packet data at the doubled data rate in step 132. Here, if the AT is under power limitation, it maintains the current data rate.
As a result of transmission of the RAB with an FAB in time multiplexing on a forward MAC channel being a common channel, ATs receiving the RAB increase or decrease their data rates indiscriminately.
Despite bandwidth control and overhead control, the above conventional reverse data rate controlling method in the HDR system cannot ensure data transmission quality due to the indiscriminate control without considering AT and packet characteristics. Therefore, there is a need for an individual reverse data rate control for ATs and a bandwidth and overhead control based on the individual data rate control.
In the existing HDR system, if an AT receives RAB=1 on a forward MAC channel from at least one AN in the active set, it always reduces its data rate by half unless the reverse data rate is below 19.2 kbps. On the contrary, if all ANs in the active set transmit RAB=0, the AT doubles the data rate. In order to increase the twofold data rate by two times, the AT must receive forward MAC channels with RAB=0 from all ANs of the active set. Thus, the reverse data rate is monotonously increased. When the AT transmits packets requiring real time transmission or high quality transmission, an AN should allow the AT a data rate two or more times as high as the current data rate in a reverse margin band. In other words, the reverse data rate control must be performed on an individual AT basis.